Luster pigments have a unique appearance. They have a pearlescent appearance which is pleasing to the eye. Those luster pigments which have been widely commercialized are mica flakes coated with a metal oxide such as titanium dioxide and/or ferric oxide. They are described in U.S. Pat. Nos. 3,087,828 and 3,087,829. The pigments are recommended for use in many formulations and have found wide acceptance in automotive paints, printing inks, plastic bottles, cosmetics and simulated pearls. Other luster pigments which are commercially available, though have not been as widely used, have an inorganic platelet-like particle such as glass with the metal oxide coating. They are described in U.S. Pat. No. 3,331,699. Organic luster pigments, as described in U.S. Pat. No. 5,026,429, are also available.
Use of the luster pigments in vitreous compositions, however, has been limited. Vitreous compositions, per se, are well known. They are based on ground glass, glass forming materials or a mixture of both. Such compositions can be coatings which are applied to a substrate and then heated or fired to a temperature sufficient to cause the coating components to melt. When cooled, a thin coating of glass is formed on the substrate. The coatings are commonly referred to as vitreous enamels when the substrate is a metal and vitreous glazes when the substrate is a ceramic. The coatings are used to decorate the substrate with color or artistic renditions as well as add to the substrate's durability in terms of scratch and mar resistance, moisture barrier, etc.
The customary and normal procedure for incorporating pigments in vitreous compositions is to include them in the body of the composition's glass component. Alternatively, the pigments are incorporated in a vitreous flux which is applied to a vitreous substrate. In either case, the vitreous composition must be heated to a high temperature to fuse the glass components. It follows that any pigment included in the formulation must be able to withstand the high temperatures. This has not been a problem with many of the pigments which are available. Data sheets provided by the pigment suppliers as well as acquired knowledge of the artisan or manufacturer are sufficient. However, understandably there are certain pigments either because of their color or other visual effect which cannot be used in certain vitreous compositions. It has been found that the luster pigments are not stable at the elevated temperatures experienced. It appears that the glass components attack the pigments at the elevated temperatures. The formed surfaces simply do not have the desired appearance.
The instability problem of luster pigments in vitreous compositions, including coatings and substrate bodies is recognized. U.S. Pat. No. 5,022,923 acknowledges the problem and suggests a solution. The described pigments are provided with a top coating of tin dioxide and/or cerium dioxide. Necessarily, the specially treated luster pigments are more expensive. Additionally, while more stable, their stability is less than ideal. Certain applications of the disclosed luster pigments still do not result in the desired appearance. Furthermore, when the luster pigments are incorporated in a vitreous flux, an inordinate amount of luster pigment is required to obtain proper color intensity. Another drawback is the fact that transparent fluxes must be used for the desired color effects.
In accord with a need, there has been developed methods of imparting a desired luster pigment appearance to substrates having a vitreous surface. The application methods are economical and practical. The resultant products produced by the methods have the desired pleasing appearance with no disadvantages.